The present invention relates in general to strip chart data recorders, to strip chart temperature recorders and, more specifically, to strip chart recorders in which the chart's mechanical drive mechanism may be subjected to adverse temperature conditions and to wide variations in ambient temperature.
In recorders of the type described above, it is highly desirable that the accuracy of the chart drive not be affected by the ambient temperature. For example, in strip chart recorders which are used to monitor ambient temperatures using, for example, a temperature responsive recording stylus, it is necessary that the very ambient temperature which the device monitors not preclude accurate operation.
A continuous or strip chart temperature recorder which combines inexpensive construction with accuracy is described in my U.S. Pat. No. 4,007,637, issued Feb. 15, 1977. This patent is hereby incorporated by reference in its entirety. The continuous chart temperature recorder described in the referenced U.S. Pat. No. 4,007,637 patent is a disposable recorder for measuring ambient temperature over long periods of time (days or weeks) without maintenance or attention and, thus, is ideal for applications such as monitoring the temperature in refrigerated trucks or railroad cars.
The disposable continuous chart temperature recorder covered in the referenced U.S. Pat. No. 4,007,637 patent uses a spool-to-spool tape-type chart for recording temperature as a function of time. The chart or tape is unwound from a feed roll or spool onto a take-up roll which is driven by a spring. The force and angular velocity of the coil torsion spring decreases as the coil unwinds. This compensates for the increasing diameter and speed of the chart on the tape-up roll and provides relatively constant chart speed. Alternatively, the chart's markings in units of time along its length can be tailored to changes in speed as the spring unwinds and the chart winds.
A temperature-operated stylus records ambient temperature on the moving chart and a speed governing gear train is connected to the feed spool or the take-up spool and is operative to govern the speed of travel of the spools. Specifically, the controlling unit or governor comprises a multiple gear, speed-reducing gear train, one end gear of which is connected to the shaft of the unwind spool. The gear at the opposite end of the train is meshed with a paddle wheel. Both the gear train and the paddle wheel are mounted in a housing containing a viscous liquid. The drag due to the viscous liquid helps to reduce the speed of the gear train and aid speed control.
Despite its relatively inexpensive construction, the disposable-type temperature recorder disclosed in my referenced U.S. Pat. No. 4,007,637 patent provides useful temperature monitoring over a wide range of ambient temperatures. This accuracy requires the use of a viscous fluid such as Dow-Corning #200 Fluid which has a relatively good (flat) temperature to viscosity profile. However, the viscosity of such fluids does undergo change when subjected to wide ranges of temperatures.